Improving the Efficiency of Colchicine-Based Chromosomal Doubling of Maize Haploids

Production and use of doubled haploids (DH) is becoming an essential part of maize breeding programs worldwide as DH lines offer several advantages in line development and evaluation. One of the critical steps in maize DH line production is doubling the chromosomes of in vivo-derived haploids so that naturally sterile haploids become reproductively fertile diploids (DH) to produce seed. This step of artificially doubling the chromosomes is labor-intensive and costly; hence, optimizing protocols to improve the doubling success is critical for achieving efficiencies in the DH production pipelines. Immersion of 3–4-day old germinating haploid seedlings in colchicine solution is commonly used for chromosome doubling in large-scale maize DH line production. This manuscript presents a new method of colchicine application to haploid seedlings that showed superior doubling rates compared to other methods like standard seedling immersion, seed immersion, root immersion, and direct application of colchicine solution to the seedlings at V2 stage in the greenhouse trays. The new method involves immersing the crown region of the haploid seedlings along with all the seedling roots at V2 stage in the colchicine solution. Further experiments to optimize this method indicated that increasing colchicine concentration had a very positive effect on overall success rate in chromosomal doubling, while not drastically affecting survival rate. The optimized method showed on average 5.6 times higher overall success rate (OSR) compared to the standard haploid seedling immersion method which was the second-best method in our experiments. This improved method of colchicine application saves resources by reducing the number of haploids to be generated and handled in a maize DH production pipeline

Effectiveness of <i>R1-nj</i> Anthocyanin Marker in the Identification of In Vivo Induced Maize Haploid Embryos

Doubled haploid (DH) technology has become integral to maize breeding programs to expedite inbred line development and increase the efficiency of breeding operations. Unlike many other plant species that use in vitro methods, DH production in maize uses a relatively simple and efficient in vivo haploid induction method. However, it takes two complete crop cycles for DH line generation, one for haploid induction and the other one for chromosome doubling and seed production. Rescuing in vivo induced haploid embryos has the potential to reduce the time for DH line development and improve the efficiency of DH line production. However, the identification of a few haploid embryos (~10%) resulting from an induction cross from the rest of the diploid embryos is a challenge. In this study, we demonstrated that an anthocyanin marker, namely R1-nj, which is integrated into most haploid inducers, can aid in distinguishing haploid and diploid embryos. Further, we tested conditions that enhance R1-nj anthocyanin marker expression in embryos and found that light and sucrose enhance anthocyanin expression, while phosphorous deprivation in the media had no affect. Validating the use of the R1-nj marker for haploid and diploid embryo identification using a gold standard classification based on visual differences among haploids and diploids for characteristics such as seedling vigor, erectness of leaves, tassel fertility, etc., indicated that the R1-nj marker could lead to significantly high false positives, necessitating the use of additional markers for increased accuracy and reliability of haploid embryo identification